Generally, the luminous efficacy of a fluorescent lamp is about 80 luminance/watt, which is approximately five times of the conventional incandescent lamp (e.g. 16 luminance/watt). The average life of the fluorescent lamp is about 6,000 hours, which is approximately three times of the conventional incandescent lamp (e.g. 2,000 hours). In addition, heat generated from the incandescent lamp is about 61% of the input power. In contrast, heat generated from the fluorescent lamp is about 37% of the input power, which is much less than that of the incandescent lamp. Since the fluorescent lamp has many advantages over the incandescent lamp, the fluorescent lamp is widely used in offices or homes.
As known, in a case that the lamp tube of the fluorescent lamp is driven to illuminate at a high frequency, a higher light output ratio is achieved when identical output power is applied. In other words, such illuminating approach is less power consuming. With increasing development of electrical and electronic technologies, electronic ballasts are widely employed to replace the starters of the fluorescent lamp system and the conventional ballasts.
Referring to FIG. 1(a), a schematic circuit diagram of a conventional electronic ballast is shown. The electronic ballast 10 is not preheated but uses a push-pull parallel resonant circuit. The electronic ballast 10 is powered by a DC power supply 11 and comprises a push-pull parallel resonant circuit 14 including an output transformer 12, a fluorescent lamp 13 and two switching elements Q1 and Q2. By controlling the turning on/off statuses of the switching elements Q1 and Q2, the DC voltage provided by the DC power supply 11 is converted into a high-frequency AC voltage so as to activate several sets of fluorescent lamps 13. The winding T1 is used to monitor a current change in the primary winding of the output transformer 12 so as to control the operation of the switching elements Q1 and Q2, where the winding T1 is a part of the output transformer 12.
The electronic ballast 10 of FIG. 1(a) is popular because of some advantages. For example, the mechanism for driving the circuit is simple. In addition, the lamp tubes can be used in parallel. Since both output ends of the electronic ballast can be operated in an open or close circuit mode, no additional protection circuit is required. The lamp tube may be lighted without restarting the electronic ballast when the lamp tube is exchanged. Afterward, the mechanism for starting the electronic ballast is diverse and includes for example rapid start, instant start and program start. On the other hand, the electronic ballast 10 still has some drawbacks. For example, the switching elements Q1 and Q2 have large voltage stresses, the production process of the output transformer 12 is complicated, the volume of the inductor T2 is bulky, and so on. In views of the large voltage stresses of the switching elements Q1 and Q2, the node voltage V1 between the inductor T2 and the parallel resonant circuit 14 will has large transient voltage induced by the parallel resonant circuit 14 at the instant moment when the electronic ballast 10 is started. The transient voltage is slowly reduced and then reaches a steady state. Since the collector-to-emitter voltage (VCE) for the switching element Q1 or Q2 and the node voltage V1 is in a linear relationship. If the node voltage V1 is very large, the switching elements Q1 or Q2 is subjected to a large voltage at the instant moment when the electronic ballast 10 is started, as shown in FIG. 1(b).
Accordingly, if the switching element Q1 or Q2 is not conducted, the region between the collector and the emitter thereof should sustain a large output voltage. Otherwise, the switching element Q1 or Q2 may have a breakdown, and thus the electronic ballast 10 fails to normally function. For example, when the electronic ballast 10 is started, the switching element Q1 is conducted but the switching element Q2 is shut, the region between the collector and the emitter of the switching element Q2 should sustain a large transient voltage generated from the node voltage V1. In contrast, when the switching element Q2 is conducted but the switching element Q1 is shut, the region between the collector and the emitter of the switching element Q1 should sustain a large transient voltage generated from the node voltage V1. Generally, the switching element Q1 or Q2 used in the electronic ballast 10 is a transistor capable of sustaining a high voltage such as 1.6 KV (according to the specification of Bipolar Junction Transistor, the sustainable largest transient voltage of VCE is 1 KV or 1.6 KV). However, these transistors are low in selectivity and not cost-effective.
In order to have the region between the collector and the emitter of the transistor sustain a large transient voltage, another conventional electronic ballast is developed, as can be seen in FIG. 2(a). The electronic ballast 20 of FIG. 2(a) uses a clamping circuit 22 for limiting the node voltage V2 between the inductor T2 and the parallel resonant circuit 24 at the moment when the electronic ballast 20 is started. The node voltage V2 has large transient voltage induced by the parallel resonant circuit 24 at the instant moment when the electronic ballast 20 is started. In such manner, the collector-to-emitter voltage (VCE) for the switching element Q1 or Q2 is reduced, and thus the damage probability of the switching element Q1 or Q2 is reduced. For example, if the node voltage V2 is 1.6 KV at the moment when the electronic ballast 20 is started, the clamping circuit 22 connected to the inductor T2 will limit the node voltage V2 from 1.6 KV down to about 1 KV, wherein the clamping circuit 22 can be a transient voltage suppressor such as Type P6KE400A available from ST Microelectronics or Type P6KE400A available from VISHAY. Accordingly, the transistor to be used in the electronic ballast 20 may have a lower rated voltage, for example 1.0 KV or less, wherein the transistor can be a transistor Type BUL1102E available from ST Microelectronics or Type BUJ403A available from Philips. As known, this transistor has higher selectivity and is cost-effective. In addition, the switching speed of this transistor is very fast and the switching loss thereof is small.
Since the clamping circuit 22 limits the node voltage V2 at the moment when the electronic ballast 20 is started, the reduced voltage is converted into the current Iz, as shown in FIG. 2(b). In a case that the switching element Q1 is shut but the switching element Q2 is conducted, the current Iz may flow through the emitter E and the collector C of the switching element Q2. At the moment when the switching element Q2 is conducted, the current generated from the clamping circuit 22 is very large, and thus the reverse voltage between the base B and the emitter E of the switching element Q2 may exceed the maximum allowable range. Under this circumstance, the switching element Q2 may be damaged or the average life thereof may be decreased, and the performance of the electronic ballast 20 is impaired.
FIG. 2(b) is a timing waveform diagram illustrating the current Iz from the clamping circuit, the emitter-to-base voltage (VEB) and the collector-to-emitter voltage (VCE) of the switching element Q1 or Q2. An example of the switching elements Q1 or Q2 is a BUL1102E transistor commercial available from ST Microelectronics. The maximum reverse voltage between the base B and the emitter E of the switching element Q1 or Q2 is 12V, i.e. the maximum allowable range of VEB is 12V. The maximum reverse voltage between the collector C and the emitter E of the switching element Q1 or Q2 is 1.1 KV, i.e. the maximum allowable range of VCE is 1.1 KV. As shown in FIG. 2(b), when the electronic ballast 20 is started, the transient response of the collector-to-emitter voltage (VCE) of the switching element Q1 or Q2 is limited below 1.1 KV, and the clamping circuit 22 outputs the current Iz. Since the current Iz is very large at the instant moment when the electronic ballast 10 is started, the reverse voltage between the base B and the emitter E of the switching element Q1 or Q2 exceeds 12V. Accordingly, the switching element Q1 or Q2 is readily damaged and the average life thereof will be shortened.
Accordingly, the above-described prior art electronic ballasts are not perfect designs and have still many disadvantages to be solved. In views of the above-described disadvantages resulted from the conventional electronic ballasts, the applicant keeps on carving unflaggingly to develop an electronic ballast according to the present invention through wholehearted experience and research.